Tree species in the riparian zone of bayous, streams and rivers, have elevation distributions that are dictated by their flood tolerance. Downstream of a dam, the stream usually has higher base flow and loses peak flows as compared to undammed streams in the same watershed, which alters tree species distributions in the riparian zone. Big Cypress Bayou (dammed, altered flow) and Little Cypress Bayou (undammed, natural flow) are two of the major tributaries to Caddo Lake, a Ramsar Wetland of International Importance in northeastern Texas. Along these two waterways, we established 10-m wide belt transects perpendicular to the stream bank, extending from the stream edge upslope to the upper limit of riparian and bottomland tree species and into the lower edge of upland mixed hardwood and pine species. For each individual tree greater than 5 cm in diameter, we identified the species and measured the diameter at 1.4 m height. We surveyed the elevation at the base of each tree to determine height above sea level and height above the base flow of the stream.
Results/Conclusions
Preliminary results show that at the Big Cypress Bayou site below the dam, the tree species had distributions at overlapping successive intervals upslope of the base flow of the stream. As expected, tree species were distributed by elevation: bald cypress (Taxodium distichum) and water elm (Planera aquatic) at the lowest elevation, overcup oak (Quercus lyrata) and water hickory (Carya aquatica) at mid elevations, and willow oak (Q. phellos) and water oak (Q. nigra) at the highest elevation above the stream. Smaller (younger trees) of each species had distributions at lower elevations than the larger (older) trees (some probably old enough to pre-date the dam), indicating a possible relocation of species’ distributions downslope. We hypothesize that along the undammed Little Cypress Bayou, the distributions of young and old individuals of each species will have a more similar distribution in relation to elevation above base flow. The Little Cypress Bayou riparian zone will be sampled during Summer of 2018. We will also core all the trees along both streams to calculate the age of each tree. Collectively, our results will help define the extent of alteration of the riparian zone along the dammed Big Cypress Bayou and guide decisions about the magnitude of controlled dam releases to restore more natural flow regimes.